CAN A SINGLE AGB STAR FORM AN AXIALLY SYMMETRICAL PLANETARY-NEBULA

We apply a method, which is traditionally used for the solar magnetic field, to estimate the magnetic activity of AGB stars. We find that any magnetic-field model which tries to explain axisymmetrical mass loss from single AGB stars encounters severe difficulties. This order-of-magnitude calculation suggests that magnetic activity in AGB stars could be significant only if the envelope is spun up by a binary companion. But then other effects due to the companion are likely to be more dominant than the magnetic activity. We then conduct a preliminary study of a possible mechanism by which a single star might lose mass axisymmetrically. The results suggest that as the envelope mass of an AGB star decreases to less than or similar to 0.1M. the nature of the fundamental mode excitation could change. It is possible that, due to the same mechanism, higher-order radial modes and nonradial p modes could become significant to the mass-loss process when the envelope mass becomes very low. We argue, however, that even this mechanism, if it works, requires a binary companion to spin up the AGB envelope.